This invention relates to a method of and apparatus for quenching a hot metal object.
It is very well known in the art of heat treating metal that quenching a metallic object (that is, rapidly cooling the object from a heat treatment temperature, typically at least 850° C., to a much lower, usually room, temperature) can significantly improve its mechanical properties and characteristics. For example, quenching can be used to harden the object and/or to improve its mechanical properties, by controlling internal crystallisation or precipitation, or both. Traditionally, quenching has been carried out using liquid such as water, oil or brine, either in the form of an immersion bath or a spraying medium. In more recent years, gas quenching methods have been developed. Gas quenching has the advantage of not usually requiring an after quenching step to clean or wash the quenched metal object. Another advantage of gas quenching is that if an oil or water-based fluid is used non-uniformity problems can arise as a result of Leidenfrost's phenomenon, whereas in gas quenching, this problem is believed not to arise.
The main drawback of the gas quenching which, has to now been limited in its commercial use, is a difficulty in achieving a quench rate comparable to those that characterise liquid-based cooling methods. Gas quenching is discussed in “Innovations in Quenching Systems and Equipment: Current Status and Feature Developments”, by F T Hoffmann et al, Heat Treatment of Metals, 1999, 3, pp 63 to 67. Hoffmann et al dedoes not however disclose how to form a hydrogen quenching atmosphere.
Gas quenching is also disclosed in EP-A-0 911 418 and in U.S. Pat. No. 5,770,146. GB-A-1 394 197 describes the operation of a furnace for annealing coiled steel strip. The furnace has a series of five cooling sections which employ recycled gas from the annealing section. The recycled gas is coded and supplied to the cooling sections by means of jet nozzles. A ROOTS-type blower may be used to recirculate the gas from the annealing section to the nozzles. Cooling rates of up to 25° C. per hour are achieved. Such cooling rates are to be contrasted with the high cooling rates of at least 50° C. per hour that characterise gas quenching.